To meet upcoming diesel emissions regulations, a 4-way catalyst exhaust after-treatment system has been widely accepted for light-duty and heavy-duty diesel engines. The 4-way catalyst system is composed of a diesel Oxidation Catalyst (DOC), a diesel particulate filter (DPF), and a lean NOx trap (LNT) or selective catalytic reduction (SCR) device. The DPF can be catalyzed or non-catalyzed. This combination of devices is called a “four-way catalyst” system because in addition to converting carbon monoxide, hydrocarbons and nitrogen oxides, it reduces the amount of soot particles, as a fourth component.
In a 4-way catalyst exhaust after-treatment system, performance of each component is significantly dependent on its temperature. In particular, a LNT requires a certain temperature window as well as favorable upstream exhaust compositions to obtain optimal NOx adsorbent capacity. However, conventional 4-way catalyst exhaust after-treatment systems cannot meet LNT temperature requirements under all engine operating conditions.